Patients with stable coronary artery disease may experience episodes of myocardial ischemia during daily life. The mechanisms underlying the occurrence of these episodes are not well understood. To investigate this issue, we performed ambulatory ECG monitoring in 54 patients with stable coronary artery disease and positive exercise test. The ischemic threshold was measured from the exercise test as heart rate at the onset of ST-segment depression. An increase in heart rate preceded the vast majority (89%) of the ischemic episodes during monitoring. The number of episodes significantly correlated with ischemic threshold, and even more strongly with the number of times that the patients achieved ischemic threshold during monitoring. These findings indicate that the most important mechanism in the genesis of episodes of myocardial ischemia during daily life in patients with coronary artery disease is the increase in myocardial oxygen demands as evidenced by the increase in heart rate. The frequency of episodes of myocardial ischemia during daily life is determined by the frequency with which the ischemic threshold is reached during the day.